ATLANT. DEEP-SEA EXPED. 1910. VOL. III]. 



SCYPHOMEDUSAE. 



15 



plete. Indeed, the table tells us that the disposition 

 of the growing stages is the same in both groups of 

 variants. 



The groups of different colouring also show the same 

 bathymetrical arrangement in both groups, and thus there 

 is no reason for a separation of A. Verrilli and A. Bairdi 

 as biological forms. 



The occurence of the variant groups as illustrated by 

 the data from the cruise of the "Michael Sars" at once 

 gives us the answer to the question whether we have 

 two geographical forms or not. The accompanying table 

 gives no evidence whatever that the two "species" are 

 confined to special currents or regions, and we have thus 

 no right to consider A. Bairdi and A. Verrilli as geogra- 

 phical forms. 



We may now ask: What is the cause of the more 

 or less distinct radial furrows in a great many specimens 

 of Atolla Bairdi? In most cases where radial furrows 

 could only be traced at the margin of the central disc, 

 a slight pressure with the point of the forceps in the 

 middle of the disc will give rise to distinct and regular 

 radial furrows all over the disc. This seems to indicate 

 that the radial furrows of A. Bairdi are due to contractions 

 of the central disc. From this we might infer that the 

 central discs of typical specimens of Atolla Verrilli are 

 relatively smaller than typical specimens of A. Bairdi. 

 An examination of the accompanying table (fig. 8) proves 

 this to be the case; the central disc of Atolla Verrilli is 

 generally a little smaller than that of A. Bairdi. This 

 affords further evidence that the radial furrows of the 

 central disc in A. Bairdi are, at least partly, due to 

 contraction. 



Atolla Bairdi is a deep-water medusa and, in the 

 Atlantic Ocean prefers a depth of about 1000 metres. 

 Its upper limit must be drawn a little above 500 metres 

 (see table fig. 9), although a single specimen was taken 

 at a depth of 250 metres. Beyond 1000 metres the number 

 of specimens rapidly decreases, and in catches from greater 

 depths than 1500 metres only single specimens were ob- 

 served. It is, of course, possible that these single speci- 

 mens from the deep hauls may have been caught on the 

 way to the surface, but this does not affect what has been 

 said of the general conditions of the species. During the 

 cruise of "Michael Sars" no difference could be observed 

 in the bathymetrical distribution of the individuals at 

 northern and southern stations. 



The pigmentation of the individuals is subject to 

 great variations. Many specimens are quite hyaline with 

 the exception of an almost black stomach; nearly the 

 entire subumbrella of other individuals is brownish black, 

 and in such specimens even the greater part of the exum- 



brella may be darkly pigmented. Of course every transition 

 stage is found between the extremes. An interesting 

 question concerning such typical inhabitants of great 

 depths is as to whether the colours depend upon the 

 bathymetrical occurrence of the individuals. For the pur- 

 pose of solving this problem four groups of different 

 colouring were kept separate in our material (see the 

 accompanying table). The stages or groups have been 

 limited as follows: — I only the stomach and occassionally 

 the gonads containing pigment; II the ringmuscle also 

 pigmented; III pigment covering other parts of the subum- 

 brella too, but the gonads always visible from the exum- 



Depth 

 (meters) 



Stages of pigmentation 



Total 



I 



11 



III 



IV 





 250 

 500 

 750 

 1000 

 1250 

 1500 

 1500 



1 

 23 

 5 

 6 

 2 

 1 



6 



20 



36 



5 



10 



16 



21 



3 



5 



2 



7 

 3 

 4 

 4 



1 

 29 

 48 

 66 

 14 

 20 



2 



Total 



38 



77 



47 



18 



180 



Fig. 9. Table showing the bathymetrical distribution of 



the "Michael Sars" specimens of Atolla Bairdi arranged 



in the four colour-groups (I— IV) mentioned in the text. 



The figures denote number of individuals. 



brellar (upper) side of the medusa; IV pigmentation so 

 dense that the gonads are quite invisible from the upper 

 (exumbrellar) side of the medusa. 



Although the absolute maxima of the two intermediate 

 groups are situated at the same depth, we see the same 

 displacement of the bulk towards deep water, which is 

 still more pronounced from stage I to stage II, where even 

 the absolute maximum has become displaced. There is 

 no distinct difference to be seen between stages III and 

 IV. On the whole, the same rule holds good regarding 

 the pigmentation of Atolla Bairdi as in the case of Peri- 

 phylla hyacinthina (see p. 6), the pigmentation increasing 

 towards deep-water. In species which live at great depths 

 like Atolla Bairdi the less pigmented, or almost completely 

 hyaline, specimens have their optimum in the shallower 

 layers the densely pigmented individuals, on the contrary, 

 occurring in the deeper waters. 



The cruise of the "Michael Sars" has, on the whole, 

 not increased the geographical range of Atolla Bairdi, 

 which was previously known from many parts of the At- 

 lantic Ocean even north of the Wyville Thomson ridge. 



